1. Field of the Invention
The present invention relates to a method for forming a silicon-containing crystalline semiconductor film applied to a semiconductor device such as Thin Film Transistors (hereinafter simply referred to as “TFT”).
2. Description of the Prior Art
The recent years have seen a rapid advance of a technology wherein a semiconductor circuit is formed by constructing a TFT on an insulative substrate such as a glass substrate. The technology is used for fabricating electro-optical apparatus such as active-matrix liquid crystal displays. The active-matrix liquid crystal display means a monolithic liquid crystal display having a pixel matrix circuit and a driver circuit formed on the same substrate. The above technology is also used for the development of a System On Panel incorporating logic circuits including γ-compensation circuit, memory circuit, and clock generating circuit.
Since such driver circuit and logic circuits must provide high-speed performances, it is improper to apply an amorphous silicon film to a semiconductor film as an active layer of the TFT. Under the current circumstances, TFTs comprising a semiconductor layer of polycrystalline silicon film are becoming predominant. As to the substrate on which the TFT is formed, the use of the less costly glass substrate is demanded. Hence, the development of low temperature process applicable to the glass substrate has become brisk.
As the low temperature process techniques, there is known a technique including the steps of introducing a catalyst element effective to accelerate crystallization, such as nickel (Ni), into an amorphous silicon film, and performing heat treatment for formation of a crystalline silicon film. It is known that the crystallization can be accomplished by heat treatment at temperatures of 550 to 600° C., which are below the temperature resisted by the glass substrate. The crystallization technique requires the introduction of the catalyst element into the amorphous silicon film. Examples of a suitable introduction method include plasma CVD process, sputtering, evaporation, and spin coating.
A test was conducted on the plasma CVD process using a nickel element as a typical catalyst element for accelerating crystallization for examining the crystallization process of the amorphous silicon film. As a result, the following facts were found.
Fact 1: Where the nickel element is introduced into the amorphous silicon film by plasma CVD process, the nickel element has been introduced into a substantially deep portion of the amorphous silicon film before the heat treatment is performed.
Fact 2: In the crystallization, initial nuclei are generated from a surface of a region doped with the nickel element.
Fact 3: Where the nickel element is introduced into the amorphous silicon film by plasma CVD and the crystallized crystalline silicon film is irradiated with laser light, excessive nickel element is deposited on a surface of the crystalline silicon film.
It is inferred from these facts that all the nickel element doped by plasma CVD does not effectively work and that the nickel element present in a contact surface portion between the nickel element and the amorphous silicon film contributes to the low temperature crystallization.
Although the catalyst element for accelerating crystallization is necessary for the crystallization of the amorphous silicon film, it is preferred for the catalyst element to be present in minimum possible concentrations after the crystallization. For one reason, the catalyst element is essentially a metal element which forms a deep energy level in the resultant crystalline silicon film, trapping carriers. Hence, the catalyst element may adversely affect the electrical characteristics and reliability of a TFT if the TFT is constructed using the crystalline silicon film. For another reason, it is practically confirmed that the catalyst element present in the crystalline silicon film is segregated in the grain boundary. Thus, it is inferred that the catalyst element may be a causative factor for a sporadic increase in the OFF current of the TFT (current through the TFT in OFF state).
For the above reasons, it is required to uniformly introduce the catalyst element (e.g., nickel element) into the place near the surface of the amorphous silicon film in a range to permit the low temperature crystallization and at minimum possible concentrations. In principle, therefore, the catalyst element introduction method wherein the catalyst element enters deep into the amorphous silicon film is improper. On this account, the sputtering and vapor deposition processes, similarly to the plasma CVD process, are also considered to be improper.
In this connection, there has been developed a method for effectively introducing the catalyst element only into a place near the surface of the amorphous silicon film, wherein a solution containing a catalyst element (hereinafter, simply referred to as “catalyst element solution”) is applied by spin coating. This method is disclosed in Japanese Patent Laid-Open No. 211636/1995. The spin coating method as a catalyst element adding technique set forth in the publication has the following features.
Feature 1: Control of the concentration of the catalyst element in the catalyst element solution provides an easy control of the amount of catalyst element to be added to the amorphous silicon film surface.
Feature 2: This permits the minimum amount of catalyst element required for crystallization to be easily added to the amorphous silicon film surface.
Feature 3: The assurance of the reliability and electrical stability of the semiconductor device dictates the need for reducing to minimum the amount of catalyst element present in the crystallized crystalline silicon film. In the case of spin coating, the minimum amount of catalyst element required for crystallization can be easily added by adjusting the concentration of the catalyst element of the catalyst element solution. This results in the prevention of excessive catalyst element introduction, advantageously assuring the reliability and electrical stability of the semiconductor device.
The spin coating of the catalyst element includes the steps of: applying dropwise a catalyst element solution onto a substrate surface thereby forming a solution puddle thereon; and spinning the substrate at high speed for throwing away the catalyst element solution so applied dropwise thereby adding a desired amount of catalyst element to the substrate surface. Having the above three merits, the spin coating process is an important technique which is now being studied for practical use. However, where the catalyst solution is directly applied to the amorphous silicon film surface, the catalyst solution has poor wettability with the amorphous silicon film, so that the catalyst element solution is repelled by the substrate surface. The solution repellency emerges in a case where the catalyst element solution is an aqueous solution (typically an aqueous solution of nickel acetate). It is understood that the repellency of the catalyst element solution by the substrate surface results from that a bonding force (hydrogen bond) linking together water molecules as solvent molecules is greater than a bonding force between constituent atoms of the substrate surface (silicon atoms) and the water molecules.
Two major countermeasures are contemplated against this problem. One is to change the solvent molecules from water molecules to molecules having a smaller bonding force such as organic molecules or otherwise to add a surface tension modifier (comprising organic molecules) to an aqueous solution as the solvent for decreasing the surface tension of a droplet of the aqueous solution. The other is to form a thin film over the substrate surface for producing a state where a bonding force between the thin film and the water molecules becomes greater than the bonding force linking together the water molecules. The former approach is fundamentally unfavorable because the surface tension modifier comprises organic molecules rich in carbon atoms, involving the likelihood that the carbon atoms as impurities may be captured in the crystalline silicon film in the subsequent thermal crystallization process.
Accordingly, the latter approach has been currently adopted wherein a silicon oxide film is formed on the surface of the amorphous silicon film. The method for forming the silicon oxide film may be a process for depositing the silicon oxide film by CVD and the like or a process for lightly oxidizing the amorphous silicon film surface (mild oxidation). If the silicon oxide film is great in thickness, the catalyst element added to the surface may be blocked by the silicon oxide film so as to be inhibited from effectively working in the crystallization of the amorphous silicon film. Hence, a demand exists for a silicon oxide film having enhanced wettability with the catalyst element solution and such a ultra-thin film as not to block the catalyst element.
A preferred method for forming such a ultra-thin silicon oxide film is a treatment using ozone water (also called hydroxyl radical water) in the light of throughput and convenience. The ozone water treatment has been practically used for improving the catalyst element solution in wettability with the amorphous silicon film. It is possible to form a ultra-thin silicon oxide film of not more than 5 nm in thickness over the amorphous silicon film surface by treating the amorphous silicon film surface with the ozone water for about 60 seconds. In the formation of the ultra-thin silicon oxide film by the ozone water treatment, the thickness of the silicon oxide film depends much more upon the concentrations of ozone present in the ozone water than treatment time. It is also known that because of the effect of the diffusion controlled rate of the oxide, the formation of the silicon oxide film hardly proceeds beyond the thickness of 5 to 10 nm. Therefore, the ozone water treatment is featured by the capability of forming uniform ultra-thin silicon oxide films suffering less thickness variations in a case where the treatment is performed in a saturated sate to provide a stable thickness of the silicon oxide film.
For these reasons, the process for forming the ultra-thin silicon oxide film by the ozone water treatment has been used in the practical procedure as a preparatory step to the spin coating of the catalyst element solution. A procedure of forming a crystalline silicon film, including the ozone water treatment includes in this order: deposition of an amorphous silicon film; removal of a natural oxide film on the amorphous silicon film surface; ozone water treatment for forming a ultra-thin silicon oxide film over the amorphous silicon film; spin coating of a catalyst element solution; and heat treatment for thermal crystallization of the amorphous silicon film.
However, when the ozone water treatment for forming the ultra-thin silicon oxide film on the amorphous silicon film surface was applied to the actual semiconductor film forming procedure, the following serious problem occurred. In some of the substrates of a lot, a reddish eddy pattern was observed on the thermally crystallized crystalline silicon film. The reddish eddy pattern indicates a serious problem that the substrate surface formed with the crystalline silicon film contains a local amorphous region of lower crystallinity.